One of the first applications of using radio waves to produce heat was in the area of medical diathermy or the heating of muscle and skin tissue by radio waves. Radio waves were used in this context for the treatment of arthritis at the beginning of the century. In the adhesives industry, the first radio frequency cure application with water based adhesives dates back to the late 1930's in the lumber industry. At this time, boards were being pressed with steam heated platens into plywood sheets. Veneers were being hot pressed into curved shapes with hand tool metal molds. Switching to radio frequency heating equipment saved money, time, energy and materials, by eliminating the steam platens. This new technology was immediately accepted by the wood working industry.
The rapid radio frequency cure of plywood was invaluable during World War II. Examples of applications for radio frequency cured water based adhesives in plywood products include the P.T. boat and the British mosquito bomber airplanes. The radio frequency cure industry continued to grow rapidly in the 1950's. The wood and lumber industries converted more and more to radio frequency. Plastic welders, heat sealers and thermoplastic preheaters were developed. Rubber vulcanizing systems and compression molding techniques for the tire industry were also developed.
A representative sample of the present state of the art includes Haven et al, U.S. Pat. No. 4,423,191, which teaches the curing of thermoset resins such as polyurethanes, phenolics, polyesters, and epoxies through the use of dielectrically lossy particles and with the application of an electric field having a frequency ranging from 1 MHz to 30 MHz and preferably about 1 MHz to 100 MHz.
Wilkinson et al, U.S. Pat. No. 4,941,936, disclose a process for using dielectric heating in the manufacture of reinforced plastic automotive body parts. Thorsrud et al, U.S. Pat. No. 4,360,607, discloses a sensitizing concentrate generally comprising four constituents including (1) polymeric alkylene glycols and their mono- and di-alkyl ethers, (2) alcohol amines and their hydrocarbyl substituted derivatives, and optionally, (3) silica, and (4) a "plastomer", i.e., an elastomer such as a butadiene-styrene copolymeric rubber. Thorsrud, U.S. Pat. Nos. 4,661,299, 4,767,799 and 4,790,965, discloses compositions intended to enhance the radio frequency sensitivity of moldable compositions such as zinc oxide, bentonite clay, and crystalline or amorphous alkali or alkaline earth metal aluminsilicate. Thorsrud, U.S. Pat. No. 4,840,758, discloses a method for preparing molded thermoplastic articles including the steps of admixing a radio frequency energy sensitizing agent, namely, N-ethyl toluene sulfonamide into a thermoplastic polymer to provide a mixture having moldable consistency. Generally, the radio frequency sensitizer is activated at 100 MHz and used in a concentration ranging from 2 to 15% of the weight of the polymer.
Pham et al, U.S. Pat. No. 4,853,420, discloses polymers which are readily susceptible to high frequency radiation, from about 0.1 to about 30,000 MHz as a means of heating these thermoplastic polymers to a pliable consistency. Beckert et al, U.S. Pat. No. 4,296,294, disclose a mechanism for drying aqueous based adhesives based on varying the energy in a radio frequency field to effectively promote the evaporation of water from the adhesive used in bookbindings. Schonfeld et al, U.S. Pat. No. 4,083,901, disclose a process for curing polyurethane elastomers using a curing agent.
Toa Gosei Chem Ind. Ltd., Japanese Patent No. 2,086,672, discloses an adhesive complex comprising polypropylene which optionally may contain an ethylene-propylene copolymer, an alpha, beta ethylenically unsaturated carboxylic acid or its anhydride, a radical initiator, and an fine particle. Maeda et al, U.K. Patent Application 2,200,128A, discloses a polyphenylene oxide resin composition suitable for high temperature dielectric applications. Gauvin et al, "Forging and R-F Assisted Processing of UHMW Polyethylene, Two Alternatives for Shorter Cycles and High Performance", disclose a comparison between radio frequency assisted processing and forging processing of thermoplastic compositions.
Radio frequency has been applied to various other adhesives with little or no commercial success. Urethanes, resin composites, epoxies, dental acrylics, and epoxy adhesives, were all subjected to experimentation of radio frequency cure. Moreover, polyesters, silicons, polyolefins generally, polystyrene, polyvinylchloride, acrylics, polycarbonates, urethanes, generally epoxies, fiberglass, are among those compounds that are not sensitive to radio frequency energy. The use of radio frequency to cure solely the adhesive composition generally failed due to a lack of appropriate equipment as well as an absence of appropriate electrical and chemical properties in the materials to be cured.